1. Field of the Invention
The present invention relates to a display device, and more particularly, to an improvement in display quality in a display device which uses thin film transistors (hereinafter referred to as TFTs) to perform display control of pixels.
2. Description of the Related Art
A display device such as a liquid crystal display device uses TFTs and the like to perform display control of pixels. As the TFTs, there are known bottom gate TFTs having a structure in which a gate electrode film is positioned on a light source side with respect to a semiconductor film. When the TFT having this structure is irradiated with light from a light source, such as a backlight, a gate electrode itself serves as a light shield mask with respect to the semiconductor film opposed to the gate electrode.
When the semiconductor film is irradiated with light, hole-electron pairs are generated. In particular, in a case of a TFT using polycrystalline silicon, frequency of the generation becomes significantly lower as carrier concentration is higher. Accordingly, in a channel region and a depletion layer region formed at a PN junction portion in a vicinity of the channel region, hole-electron pairs are more likely to be generated compared to other regions. As a result, if those regions are not sufficiently shielded against light by a gate electrode opposed thereto, hole-electron pairs are generated to generate optical leakage current, resulting in increased off-state current.
Of the TFTs, the TFT using polycrystalline silicon has a problem of a relatively large off-state current. To reduce the off-state current, for example, a multi-gate structure in which a plurality of TFTs are connected in series is employed (see, for example, JP 2002-57339 A).
In the multi-gate structure using TFTs including a gate electrode film formed on a light source side, a semiconductor film thereof includes a plurality of channel regions formed in series, with regions in which predetermined impurities are doped being interposed between the channel regions.
On this occasion, when each channel region and the vicinity thereof are irradiated with light in a state of being not sufficiently shielded against light by a gate electrode, hole-electron pairs are generated to generate optical leakage current. Accordingly, if all of the TFTs have such a structure as described above, the optical leakage current is not suppressed.
Further, it is known that optical leakage current is likely to be generated when hole-electron pairs are generated in a vicinity of an end of the above-mentioned plurality of channel regions which is positioned closest to an image signal line side or a pixel electrode side among respective both ends thereof.
The reason therefor is as follows. In the vicinity of the channel region end which is positioned closest to one of the image signal line side and the pixel electrode side, on which a higher potential is maintained, a stronger electric field is often generated than in other channel region ends. The hole-electron pairs that are generated in the vicinity of the channel region end are often separated into holes and electrons by the strong electric field, resulting in increased leakage current.
On the other hand, when each channel region and the vicinity thereof are sufficiently shielded against light by the gate electrode, optical leakage current is suppressed, but an area of the region in which the semiconductor film and the gate electrode film are opposed to each other is increased, resulting in increased parasitic capacitance. If all of the TFTs have such a structure as described above, parasitic capacitance is increased more in accordance with the number of TFTs.
When a gate voltage is turned off to maintain a pixel voltage, increased parasitic capacitance causes the pixel voltage to be largely decreased, which results in a new cause of display failure.